In the past more than 20 years burners with short but effective premixing zones (so-called EV burners: environmental friendly V-shaped burners) have been implemented in several gas turbines of the applicant, with very low NOx levels. In addition to this, three variants of premix technologies have been successfully developed and deployed into those gas turbine engines: the sequential EV burners—a technology that allows premixing of natural gas and oil into a hot exhaust stream to reheat the exhaust gases of a first high pressure turbine; the MBtu EV burners that are used to burn syngas in a premix flame with low NOx emissions; and the advanced EV burners (AEV) that are capable to prevaporize and premix liquid fuel prior to combustion and burn it with very low NOx emissions without water injection.
Document EP 0 851 172 A2 discloses an exemplary EV burner of the double-cone type, for operating a combustion chamber with a liquid and/or gaseous fuel, whereby the combustion air required for this purpose is directed through tangential air-inlet ducts into an interior space of the burner. This directing of the flow results in a swirl flow in the interior space, which swirl flow induces a backflow zone at the outlet of the burner. In order to stabilize the flame front forming there, at least one zone is provided at each sectional body forming the burner, within which zone inlet openings are provided for the injection of supplementary air into the swirl flow. Due to this injection, a film forms at the inner wall of the sectional bodies, which film prevents the flame from being able to flashback along the inner wall of the sectional bodies into the interior space of the burner.
Document EP 2 423 597 A2 shows another exemplary EV burner in the form of a double-cone burner, which has two partial cone shells which are arranged nested one inside the other, forming air inlet ducts between them, through which combustion air from the outside flows into a conical inner space of the premix burner. Linear rows of holes of injection openings, which extend transversely to the flow direction of the combustion air, are arranged on the outer walls of the air inlet ducts and through which a gaseous fuel is injected into the combustion air which flows past transversely to them.
Document DE 195 45 310 A1 discloses a further pre-mixing burner consisting of a hollow cone with an outer and inner cone casing. At least two inlet ducts run at a tangent to the inner cone casing and are positioned along a straight cone casing line. The part cone axes of the part shells formed lie on the same cone axis. The pre-mixing burner is divided into at least two, for example four, parts containing the inlet ducts so as to swirl the combustion air. A fuel nozzle is positioned at the cone tip for injecting liquid fuel.
Document WO 2009/019113 A2 describes a burner, which includes a swirl generator, a mixer, and a lance. The swirl generator encloses an inlet-side section of a burner interior and has at least one air inlet which extends tangentially with regard to a longitudinal centre axis of the burner. The swirl generator is conically designed. The respective air inlet forms a longitudinal slot along the generated surface of the cone. A plurality of such air inlets are preferably arranged in a distributed manner in the circumferential direction. As a result of this, the air can penetrate tangentially into the burner interior, as a result of which a swirl is imparted to it. The swirl generator also has a fuel inlet via which gaseous fuel can be introduced into the burner interior. The fuel inlet includes a plurality of rows of individual inlet orifices which extend along the surface line of the conical swirl generator, through which orifices the fuel gas can enter the burner interior. The mixer encloses an outlet-side section of the burner interior and has an outlet opening which is open towards a combustion space of the combustion chamber. The mixer, includes a tubular body which is connected via a tubular transition piece to the swirl generator and carries an outlet flange with an outlet opening. Via the outlet flange, the burner can be connected to the combustion chamber. The mixer is expediently cylindrically formed.
In principle, to lower NOx formation, the fuel air mixing can be extended by applying a mixing tube at the exit of a burner prior to the sudden area expansion entering the combustion chamber where the flame is stabilized by the recirculating flow. The air slots and gas channels of the current EV type conical burners are confined in the downstream part by an intersecting plane orthogonal to the burner axis. Therefore the burner swirler exit is not circular and needs a special transition piece in-between the swirler and the cylindrical mixing tube. This transition piece is prone to flash-back as it must apply a complicated 3D shape to avoid flow separation and flame stabilization in recirculation zones.
Thus, when applying a mixing tube to further enhance fuel air mixing at the downstream part of the burners, the problem consists in defining a transition piece between the swirler and the mixing tube such that recirculation zones, which could lead to flash-back, are not formed.